This invention relates generally to an electrophotographic printing machine, and more particularly concerns an improved apparatus for supporting and maintaining the transverse alignment of a moving photoconductive belt.
In an electrophotographic printing machine, a portion of a photoconductive belt is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive belt is then exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive surface selectively discharges the charge thereon in the irradiated areas. In this manner, an electrostatic latent image is recorded on the photoconductive surface which corresponds to the informational areas contained within the original document. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed by bringing a developer mix into contact therewith. The developer mix comprises toner particles adhering triboelectrically to carrier granules. These toner particles are attracted from the carrier granules to the latent image forming a toner powder image thereon. The toner powder image is then transferred to a copy sheet. Finally, the copy sheet is heated to permanently affix the toner particles thereto in image configuration. This general approach was originally disclosed by Carlson in U.S. Pat. No. 2,297,691, and has been further amplified and described by many related patents in the art.
Lateral alignment of the photoconductive belt is necessary in order to ensure that the latent image passes through each processing station at essentially the same point. Thus, it is necessary to minimize or eliminate any lateral deviations of the photoconductive belt from the pre-determined path of movement. Ideally, the photoconductive belt is perfect and its velocity vector substantially normal to the longitudinal axis of a perfect drive roller. In actuality, the velocity vector of the moving belt is not normal to the longitudinal axis of the drive roller. This may be due to the belt being skewed relative to the longitudinal axis of the drive roller, or the roller being tilted relative to the belt. In either case, the longitudinal axis of the drive roller is not perpendicular to the velocity vector of the belt. Thus, the belt tends to move laterally resulting in belt skewing. In order to minimize lateral movement, the photoconductive belt must be controlled to regulate its lateral position. Existing methods of controlling the lateral movement of the photoconductive belt include various forms of crowned rollers, flanged rollers and electrical servo systems. Systems of this type, however, frequently produce high local stresses resulting in damage to the sensitive photoconductive belt.
Edge tracking of a belt requires that the lateral forces between the flanges and belt edge be able to change the angle of approach between the belt and the next successive roller. When rollers are employed to support a belt, the lateral friction between the belt and the various rollers makes a system of this type highly impractical unless large forces applied to the belt without it being deformed, i.e. the case for unusually thick belts. Thus, it is highly desirable to eliminate or reduce lateral friction. The problem of edge guiding is thus reduced to being able to overcome the frictional and elastic forces between the belt and drive roller.
Accordingly, it is a primary object of the present invention to improve lateral tracking of a photoconductive belt employed in an electrophotographic printing machine.